The Back-End-Of-Line (BEOL) memories such as RRAM (Resistive Random Access Memory), PRAM (Phase Change Random Access Memory), MRAM have a resistive memory element that provides high speed access and non-volatility at power off. The memory cell includes a top electrode, a bottom electrode and the resistive memory element in between. The bottom electrode is connected to a control device such as a transistor or a diode. After the layers of the memory device have been patterned, the top electrode is connected to a bit line in a series of process steps collectively referred to as BEOL process which creates metal interconnect wires that are insulated by dielectric material.
A Magnetic Tunnel Junction (MTJ) resistive element for MRAM includes a free layer, a fixed layer and a barrier layer in between. A magnetic moment of the free layer is manipulated to be parallel or antiparallel to the fixed layer by applying an electric current. Whether the magnetic vector of the free layer is parallel or antiparallel to the fixed layer determines the low or high resistance state of the MTJ, which are defined as memory state “0” or “1”.
In published US patent application 20100181654 by Fujiwara, et al. (Jul. 22, 2010) an insulating film, which will be called a borazinic film herein, for a semiconductor device is described. The film is described as having low permittivity, a low leak current, high mechanical strength, stability over time, and excellent water resistance. The process for forming the film uses a carrier gas and a raw material gas, which has borazine skeletal molecules. The insulating film includes cross-linked borazine skeletal molecules and is said to have both inorganic or organic properties. A borazinic film suitable for general MRAM applications can be deposited by a CVD system such as the MAPLE (Multi Application PLasma Equipment) CVD system of Mitsubishi Heavy Industries.
After the stack of layers for the bottom electrode, the MTJ element and the top electrode have been deposited over the wafer, pads of hard mask material are formed over the selected areas for the memory cells to protect them during the etching process which forms pedestals by cutting through the entire layer stack. The present invention is directed at solving problems with the prior art etching process used to fabricate the MTJ cell pedestals.
The first issue is that a surface of the top electrode is passivated by the hard mask etchant. Once a passivation layer is formed on the surface, it affects the following top electrode etching process. It may slow the etching or may generate uniformity issues.
The second issue is top electrode erosion. Process margin for interconnecting to the bit line increases with a thickness of the top electrode. Conventional etching hard mask materials such as SiO2, Si3N4 do not have enough selectivity during MTJ cell fabrication. During the etching cycle that goes through the top electrode layer and down to the bottom electrode layer, the hard mask over the selected top electrode area is severely eroded and can be totally removed before completing the etching cycle. When the previously masked top electrode area is exposed to the etching ambient undesirable erosion occurs. Sometimes the erosion can penetrate through the top electrode to reach to the critical MTJ layers damaging the cell.
The 3rd issue is CD (critical dimension) control of MTJ stack including the electrodes. A straight sidewall profile is preferred to control the dimensions. The conventional hard mask is generally thickened to keep sufficient thickness of the top electrode for interconnect process. It requires thick photoresist to make a thick hard mask pattern, which results in a trade-off of resolution versus the thickness. The thick hard mask results in tapered sidewall shape.
The 4th issue is re-deposition during the etching cycle. Carbonaceous reactive ion etching provides a good profile of MTJ but tends to produce undesirable polymers and inhibits the following interconnecting process. It also creates a concern about reacting with the memory element and degrading the functionality. Though carbon-free etching such as ion milling produces no carbonaceous polymer, MTJ materials are re-sputtered on the sidewall which shorts the MTJ. In addition, ion milling is not preferred for fine patterning because incident angle of the ion beam to a wafer is tilted to maximize etching efficiency.
Any re-deposition materials should somehow be removed by end of the etching cycle. This disclosure provides solutions for the MTJ fabrication problems described above.